This invention relates to a time division multiple access (TDMA) system for use in carrying out communication among a plurality of earth stations through a satellite. It should be noted throughout the instant specification that the time division multiple access system is especially suitable for business satellite communication.
The satellite communication has been formerly used only in international communication. However, recent technical development enables a reduction of costs for the satellite communication and also application of the satellite communication not only to local public communication but also to so-called private business communication used in private companies.
In business communication, a wide variety of traffic, such as an image signal, a data signal, or an audio signal, must be transmitted among earth stations through a satellite and must be systematically and collectively dealt with in the form of a succession of digital signals. In addition, each earth station for the business communication is usually located in the vicinity of users, for example, on roofs of buildings of the users. In this connection, a high frequency band, such as 14/11 GHz (Ku band), 30/20 GHz (Ka band), is used to avoid an adverse influence or interference on the other microwave network systems which have already been set up.
As a general satellite communication system conveying a succession of digital signals, a Single Channel per Carrier (SCPC) system has been proposed which uses a single carrier wave at every unit channel of, for example, 64 kbps. However, the SCPC system is disadvantageous in that a maximum transmission capacity per channel is inevitably restricted to 64 kbps.
Alternatively, another proposal has also been proposed as regards a Multiple Channel per Carrier (MCPC) system which uses a single carrier wave after carrying a multiplexed signal of, for example, 1.5 Mbps, 2.0 Mbps, or the like assigned to a plurality of channels are multiplexed into a multiplexed signal of, for example, 1.5 Mbps, 2.0 Mbps, or the like. However, actual transmission traffic does not always reach 1.5 Mbps or 2.0 Mbps at each earth station. Therefore, the carrier wave may wastefully be used in each earth station.
At any rate, both the SCPC and the MCPC systems have a poor flexibility in order to systematically and collectively deal with various kinds of digital signals. This shows that the SCPC and the MCPC systems are unsuitable for the business communication mentioned before.
An improved time division multiple access system has been proposed in order to solve the above-mentioned disadvantages and shortcomings and to systematically and effectively deal with the digital signals.
In an article contributed by John E. Ohlson et al to ICC's 83 (June 19-22, 1983), and entitled "Multi-Frequency TDMA for Satellite Communications," a plurality of carrier waves can be used in a sole transponder assigned to a plurality of terminals and are time-shared by the terminals. With this structure, each terminal can transmit bursts of data at a multiplicity of time and frequency slots on a time-frequency map.
Moreover, data burst synchronization is performed without the use of a preamble. In this connection, such Multi-Frequency TDMA will be called a specific MFTDMA hereinunder. In the specific MFTDMA, demand assigned multiple access (DAMA) is readily implemented with a high frame efficiency.
The carriers in the transponder can be operated with different power levels so that high fading margins can be provided by circuit reallocation. Therefore, effective utilization of electric power is possible by controlling the power levels so as to assign a high power level to a certain one of the earth stations that receives a signal subjected to heavy attenuation.
However, the article of Ohlson et al is restricted only to the specific MFTDA. In other words, no consideration is paid to effective utilization of electric power in usual TDMA which is operated under control of a burst time plan.
The Ohlson et al article also teaches to vary power levels of the carriers in consideration of scales of earth stations and weather at each earth station in order to effectively use the electric power. However, the Ohlson et al article does not teach how to specifically vary the power levels of the respective carriers and does not even remotely suggest the necessity of varying the power levels in accordance with transmission quality requirements of respective data rates or traffic to be transmitted.